In the pulp and paper industry, there are basically two fundamentally different processing methods for converting lignocellulosic material, being wood or nonwood, into pulp used in papermaking. One processing method is chemical pulping, which uses chemicals such sodium hydroxide, sodium sulfide, sodium sulfite or different solvents, to break down bonding between each individual fiber. The other processing method is mechanical pulping, which uses mainly mechanical means such as a pair of rotating discs commonly referred to as a refiner, or a rotating grinding stone, to separate the lignocellulosic fibers from one another.
The kraft chemical pulping process (also known as the sulfate process) is typical of a chemical pulp process that produces pulps of high strength and yields of around 50%. In the kraft process the wood is chemically treated under temperature and pressure with an aqueous solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S). However, it is sometimes possible to incrementally increase the yield of the kraft process by introducing additives or chemical treatments to the process, typically before treatment with the sulfide and hydroxide. Thus, single-digit increase in yield may have a significant impact upon the profitability of a pulp mill. If a pulp mill capacity is limited due to limitations in increasing the capacity of its recovery boiler, an increase in the yield of a pulping process can increase the capacity of the mill while avoiding the limitations of the recovery system. Another chemical pulping process, where essentially no sulfur is contained in the cooking liquor, is referred to as soda cooking. In this process sodium hydroxide (NaOH), or sodium carbonate, or a combination of the two is used to produce pulp.
U.S. Pat. No. 4,012,280 discloses that improved yield of an alkaline chemical pulping process can be obtained by adding cyclic keto compounds, including anthraquinone (AQ), to the cooking liquor and treating cellulose material with the cooking liquor-AQ solution at pulping temperatures. However, in such a process the AQ additive is not recovered and is simply lost to the pulping process, even though it is known that AQ is a catalyst. U.S. Pat. No. 4,127,439 improved on the earlier AQ treatment process by limiting the exposure of cellulose material to AQ only in a pretreatment stage prior to digestion. In this process, the pretreatment liquor is separated from the cellulose material prior to digestion and the separated pretreatment liquor containing residual AQ is re-used for pretreatment. U.S. Pat. No. 4,127,439 includes the option of pretreating cellulose in a continuous process in which the treatment liquid counter-currently displaces the pretreatment liquor in a single treatment zone. However, the removal and recovery of the pretreatment liquor is limited due to the treatment in one treatment zone.
U.S. Pat. No. 4,310,383 discloses an alternative to the above pretreatment with anthraquinone in which the variation in the solubility of the anthraquinone in an alkaline liquor is used to produce an internal circulation of anthraquinone in a treatment zone. This internal circulation results from the variation in the solubility of anthraquinone which occurs in a counter-current treatment of cellulose. The AQ-containing solution is introduced at one end of a counter-current treatment zone at higher alkalinity where the AQ is more soluble. This high alkalinity is effected by also introducing highly-alkaline kraft white liquor while introducing the AQ to the cellulose. The alkalinity of the counter-current flowing liquid decreases as the alkali is consumed by the cellulose material such that the alkalinity of the AQ solution is reduced to a point where the AQ becomes insoluble and precipitates onto the cellulose. The down-flowing cellulose then carries the precipitated AQ back into the other end of the treatment zone where the alkalinity is higher such that the AQ again dissolves. The dissolved AQ then passes back counter-currently to the flow of cellulose and the cycle repeats itself. Though this process provides for the recovery and re-use of anthraquinone it is not applicable to treatments with other additives, such as polysulfide or sulfur, which are not characterized by such variation in solubility due to alkalinity.
Recently, U.S. Pat. Nos. 6,241,851 and 6,576,084 have issued which are improvements on the AQ-pulping processes as described above. Furthermore, certain bleaching sequences and bleaching stages are well-known in the art, such as those disclosed in U.S. Pat. Nos. 6,153,300 and 6,010,594 and EP 0786 029 B1.
Notwithstanding the advances in this art with respect to AQ cooking processes, some improvements are needed, especially in terms of bleaching following cooking with soda and AQ. The techniques described herein are therefore directed toward providing such improvements.